Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
21~987(~
BACKG~OUND OF THE I~v~NlloN
Field of the Invention
This invention relates to a process and apparatus
for combustion in a surface combustor-fluid heater in which
the combustion is carried out in stages within the pores of
the stationary porous bed and heat transfer is achieved
using heat exchange surfaces embP~ in the stationary
porous bed resulting in very high combustion intensity, very
high heat transfer rates, improved energy utilization
efficiency, ultra-low combustion emissions, and lower
capital and operating costs. The use of staged combustion
within a porous matrix surface combustor-heater in
accordance with this invention reduces NO% formation by the
combustion process to levels less than about 10 vppm.
DescriPtion of Prior Art
In general, heat energy may be transmitted by
conduction, convection and/or radiation. Heat transmission
by radiation and utilization of infrared energy has many
advantages over conventional heat transmissiOn by convection
and conduc'ion. The operation and construction of infrared
burners and radiant heaters is relatively simple and, thus,
more economical than other types of heat generation means.
The intensity of radiant heat may be precisely controlled
for greater efficiency and infrared energy may be focusedr
reflected~ or polarized in accordance with the laws of
optics. In addition, radiant heat is not ordinarily
affected by air currents. One type of gas-fired infrared
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generatOr currently available is a surface combustion
infrared burner having a radiating burner surface comprising
a porous refractory. The combustion mixture is conveyed
through the porous refractory and burns above the surface to
heat the surface by conduction. One such burner is taught
by U.S. Patent 1,331,022. Other surface combustors are
taught by U.S. Patents 4,666,400; 4,605,369; 4,354,823;
3,188,366; 4,673,349; 3,833,338; and 4,597,734. See also
U.S. Patent 3,738,793 which teaches an illumination burner
having a layered porous structure, the layered pores
maintaining a stable flame in a thoria-ceria illumination
burner in which co~bustion occurs not within the pores of
the combustor, but rather on the surface of the top layer.
Control of combustion emissions, in particular NOX
emissionS, is an important requirement for surface
co~bustors which are generally known to produce high
combustion intensitY and, thus, high combustion
temperatureS~ It is generally known that to reduce NOX
formation within the combustion process, it is necessary to
simultaneously remove heat from the combustion process as
combustiOn of the fuel occurs. U.S. Patent 5,014,652
teaches a fluidized bed combustion reactor/fluidized bed
cooler comprising a vertical reactor chamber designed to
contain two separate fluidized beds, one of which contains
cooling coils through which a cooling fluid is flowing to
remove heat from the bed. U.S. Patent 3,645,237 teaches a
fluidized bed water heater in which water is heated or steam
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is produced by passing water through heating coils embedded
in the fluidized bed. Similarly, U.S. Patent 4,499,944,
U.S. Patent 4,779,574, and U.S. Patent 4,646,637 teach a
heat exchanger installed in a fluidized bed. And, U.S.
Patent 4,~99,695 teaches a fluidized bed combustiOn reaction
in which heat is transferred from the fluidized bed to
water-containing tubes surrounding the reactor.
An alternative to control of NOX emissionS by
removal of heat from the combustion process as combustion of
the fuel occurs is the use of staged combustion in which the
fuel and/or oxidant are introduced into the combustion
cha~ber in stages and under conditions which maintained the
combustion temperature generally below the temperature
required for substantial NOX formation. U.S. Patent
5,080,577 and U.S. Patent 5,160,254 both teach staged
combustion within a porous matrix combustor. The '577
patent teaches a method and apparatus for low NOX combustion
in which fuel and an oxidant mixture is burned in a first
combustion zone under fuel-rich conditions and in a second
combustion zone under fuel-lean conditions. The '254 patent
teaches a porous matrix combustor with primary and secondary-
combustion zones, the primary combustion zone being
fuel-lean and the secondary combustion zone being fuel-rich
To provide the desired fuel-rich conditions in the secondary
combustion zone, fue~ and oxidant are both injected into the
porous matrix downstream of the primary combustion zone-
See also Russian Patent SU 1393994.
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U.S. Patent 5,308,473 teaches a low NOX fluidized
catalytic crac~ing regeneration process and apparatus which
utilizes dilute phase afterburning to superheat catalyst
entrained in the dilute phase region above the fluidized
bed. Similarly, U.S. Patent 5,288,397 teaches a process and
apparatuS for dense phase, at least partially co-current,
fluidized bed regeneration of a fluidized catalytic crac~er
catalyst.
One problem associated with fluidized bed
combustors is the amount of particulate matter generated by
such beds which is carried out with the products of
combustion exhausted by the combustor. In addition, the
abrasiveness of the fluidized bed particles against the
outer surfaces of heat exchangers disposed in the fluidized
bed causes erosion of the heat exchanger surfaces. Finally,
pressure drop of flow through the fluidized bed is high due
to the high flow of velocity required for fluidization.
SUMMARY OF THE INVENTION
It is an object of this invention to provide a
process and apparatus for gas fired combustion and fluid
heating which produces ultra-low combustion emissions.
It is another object of this invention to provide
a procesS and apparatus for gas fired combustion and fluid
heating having higher combustion intensity, high heat
transfer rates, and, thus, higher energy utilization
efficiency than known gas fired combustion and fluid heating
devices.
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NOx emissions are generally known to be the result
of the formation of thermal NOx, that is, NOx which is
formed from the high temperature reactions of N2 and 2 in
hydrocarbon flames, and prompt NO~, the formation of which
is absolutely dependent on the presence of hydrocarbons
while being relatively independent of temperature, fuel
type, mixture ratio, and residence time. For example, CO
and H2 flames yield no prompt NOx emissions.
Accordingly, it is an object of this invention to
provide a process and apparatus for combustion of a fuel
which reduces not only the level of thermal NOx formed in
the combustion process, but also reduces the formation of
prompt NOx emissions, compared to known combustion
processes, thereby resulting in overall NOx levelS below
about 10 vppm.
These and other objects of this invention are
achieved by a porous matrix, surface combustor-fluid heating
apparatus comprising at least one combustor chamber wall
forming a combustion chamber having an inlet end and an
outlet end, a flow dis~ributor approximately at the inlet
end of the combustion cha~ber, a stationary porous bed
disposed between the flow distributor and the outlet end of
the combustion chamber, porous bed heat exchanger means
e~bedded in the stationary porous bed, fuel/oxidant mixture
means for introducing a mixture of a fuel and a primary
oxidant into the stationary porous bed, and secondary
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oxidant means for introducing a secondary oxidant into the
s~ationary porous bed.
The porous matrix, surface com~ustor-~luid heating
apparatus in accordance with this invention is a combustion-
heat transfer device that is based on the concept of
stabilizing a combustion process and removing energy from a
porcus matrix within the same control volume. A fuel and
oxidant mixture is distributed into the porous matrix by
means of a flow distributor. In accordance with one
e~bodiment of this invention, the flow distributor is a
cooled grid. Energy from the combustion zone is radiated
and conducted throughout the porous matrix, including into a
preheat zone disposed in a region of the stationary porous
bed adjacent to and downstream of the flow distributor As
the fuel and oxidant mixture flows through the preheat zone,
the mixture is preheated to its ignition temperature,
primarily by means of convected heat transfer from the
porous matrix bodies. Due to the high turbulence levels
created by the porous matrix bodies, rapid oxidation of the
fuel occurs within the combus.ion zone. Energy from the
combustion process is convected and conducted to the porous
matrix bodies and to lower heat exchanger tubes disposed in
the lower portion of the porous matrix bed. In addition,
energy is radiated from the porous matrix bodies to these
lower heat exchanger tubes, which rapidly remove the energy
by means of a fluid ,lowing therethrough.
Energy is also radiated and conducted from the
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porous matrix bodies in the combustion zone to the porous
matrix bodies in an upper heat removal z~ne disposed in the
upper portion of the stationary porous matrix bed. This
energy, combined with convective energy from the products of
complete com~ustion, transfers energy to upper heat
exchanger tu~es disposed in the upper heat removal zone,
which upper heat exchanger tubes rapidly remove additional
energy from the stationary porous bed by means of a fluid
flowing therethrough.
Part of the uniqueness of the surface combustor-
fluid heating apparatus of this invention is the high heat
transfer surface areas produced by the porous matrix bodies
which increases heat removal rates by means of enhanced
convective and radiant heat transfer, thereby reducing
overall flame temperatures, which reduces the formation of
thermal NOx and increases fluid tube heat fluxes which, by
means of enhanced convective and radiant heat transfer,
reduces the size of the heat exchanger tubes required and
improves overall thermal efficiency. In addition, turbulent
flow is produced by the porous matrix bodies further
increasing heat removal rates by means of enhanced
convective heat transfer which reduces overall flame
temperatureS and, thus, the formation of thermal NOX,
further increasing fluid tube heat fluxes by means of
enhanced convective and radiative heat transfer which
reduces the size of the heat exchanger tubes required and
improves overall thermal efficiency, improving combustion
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intensity there~y concentrating heat release from the
combustion process, producing homogeneous combustion which
reduces peak flame temperatures which, in turn, reduces the
formation of thermal NOX, and increasing combustion
efficiency which results in low levels of unburned CO and
hydrocarbons.
Experimental, theoretical, and numerical data have
shown the products of complete combustion from the porous
matrix, surface combustor-fluid heating apparatus of this
invention without the use of staged combustion to contain
NOx levels below about 15 vppm and CO levels below about 3S
vppm ~corrected to 3% 2) and unburned hydrocarbons at less
than 5 vppm (corrected to 3% 2) In addition, heat
exchanger tube fluxes exceeding 90,000 Btu/hr-ft2 have been
demonstrated.
Introduction of the oxidant for combustion of the
fuel in stages into the porous matrix, surface combustor-
fluid heating apparatus in accordance with the process and
apparatu5 of this invention will produce NO levels below lO
vppm (corrected to 3% 2 ) '
The process for combustion of a gaseous fuel in
accordance with one embodiment of this invention comprises
introducing a fuel-rich fuel/oxidant mixture into the inlet
end of a combustion chamber in which is disposed a
stationary porous bed. The fuel-rich fuel/oxidant mixture
is burned within the stationary porous bed to form a primary
combustion zone therein. A secondary oxidant is introduced
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215987~3
into the stationary porous bed downstream of the primary
combustion zone, forming a secondary combustion zone.
Finally, heat resulting from the combustion is removed from
the combustion chamber.
BRIEF DESCRIPTION OF THE DRAWINGS
The above-mentioned and other features and objects
of this invention will be better understood from the
following detailed description taken in conjunction with the
drawings, wherein:
Fig. 1 is a schematic diagram of a gas fired,
porous matrix, surface com~ustor-fluid heater employing
single stage combustion; and
Fig. 2 is a schematic diagram of a gas fired,
porous matrix, surface combustor-fluid heater employing
staged combustion in accordance with one embodiment of this
invention.
DESCRIPTION OF PREFERRED EMBODIMENTS
Fig. 1 shows one e~bodiment of a porous matrix,
surface com~ustor-fluid heater 10 comprising at least one
wall 18 which forms a cor.~ustion chamber into which a
fuel/oxidant mixture, preferably comprising natural gas and
any gas or gaseous mixture comprising oxygen, including air,
oxygen, flue gases, nitrogen, steam, etc., is introduced
into inlet end 11 and distributed within stationary porous
bed 15 disposed in the combustion chamber by flow
distributor 13, disposed in a distributor zone defined by
arrows 14, comprising flow distributor cooling tubes 14
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2I~987Q
through which a cooling fluid is circulated to cool flow
distributor 13. Stationary porous bed lS comprises a
plurality of high temperature ceramic particles 16. Energy
from the combustion zone defined by arrows 21 is radiated
and conducted throughout stationary porous bed 15 including
into a preheat zone defined by arrows 20 disposed in
stationary porous bed 15 in the region adjacent flow
distributor 13. As the fuel/oxidant mixture flows through
preheat zone 20, the mixture is preheated to its ignition
temperature primarily due to convective heat transfer from
high temperature ceramic particles 16 comprising stationary
porous bed 15. As a result of high turbulence le~els
created by hi~h temperature ceramic particles 16 in
stationary porous bed 15, rapid oxidation of the fuel occurs
within combustion zone 21. Energy from the combustion
procesS is convected and conducted to high temperature
ceramic particles 16 and lower heat exchanger tubes 17
disposed within a lower region of stationary porous bed 15.
In additiOn, energy from high ~emperature ceramic particles
16 is also radiated to lower heat exchanger tubes 17. Lower
heat exchanger tubes 17 disposed in the lower portion of
stationary porous bed 15 rapidly remove heat from stationary
porous bed 15 by means of a cooling fluid, preferably water,
flowing therethroUgh.
Energy is also radiated and conduc~ed from high
temperature ceramic particles 16 in combustion zone 21 to
high temperature ceramic particles 16 disposed in the upper
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heat removal zone, defined by arrows 22, disposed in
stationary porous bed 15 towards outlet end 12 of combustor
10. This energy, combined with convective energy from the
products of complete combustion, is transferred to upper
heat exchanger tubes 23 disposed within stationary porous
bed 15 in the region corresponding to upper heat removal
zone 22. This, in turn, results in rapid removal of
additional energy by a cooling fluid, preferably water,
flowing through said upper heat exchanger tu~es 23.
A two-stage porous matrix, surface combustor-fluid
heater in accordance with one embodiment of this invention
is shown in Fig. 2. Two-stage porous matrix, surface
combustor-fluid heater 30 comprises at least one combustion
chamber wall 38 forming a combustion chamber having inlet
end 31 and outlet end 32. Flow distributor 33 is disposed
proximate inlet end 31 of the combustion chamber.
Stationary porous bed 35 is disposed between flow
distributor 33 and outlet end 32 of the combustion chamber.
In accordance with a particularly preferred embodiment of
this invention, stationary porous bed 35 is supported on
flow distributor 33.
Disposed within stationary porous bed 35 are
porous bed heat exchanger means comprising at least one
fluid-cooled heat exchanger tube 37 through which a cooling
fluid is flowing.
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21~9870
To provide the desired heat exchange between
stationary porous bed 35 and said at least one fluid-cooled
heat exchanger tube 37 disposed in stationary porous bed 35,
it is preferred that the outside diameter of said at least
one fluid-cooled heat exchanger tube 37 be in the range of
about 0.5 inches to about 3.0 inches. In accordance with
one preferred embodiment comprising a plurality of fluid-
cooled heat exchanger tubes 37, the ratio of tube spacing
within stationary porous bed 35 (horizontally and
Vertically)~ to the diameter of fluid-cooled heat exchanger
tubes 37 is between about 1.0 to about 3Ø
Stationary porous bed 35 comprises a plurality of
high temperature ceramic particles 36, preferably selected
from the group consisting of alumina, silicon carbide,
silica, zirconia, and mixtures thereof. In accordance with
a preferred embodiment of this invention, the mean diameter
of high temperature ceramic particles 36 is between about
0.1 and about 3.0 inches in order to provide the requisite
heat transfer and tur~ulent flow conditions within
stationary porous bed 35.
Flow distributor 33, in accordance with one
embodiment of this invention, comprises a wall having a
plurality of openings 46 through which the fuel/oxidant
mixture flows into stationary porous bed 35. To provide
cooling to flow distributor 33, at least one flow
distributor cooling tube 34 is disposed within flow
distributor 33. In a particularly preferred embodiment of
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this invention, cooled flow distributor 33 is in the form of
a membrane wall.
To provide staging of the combustion air utilized
for combustion of the fuel t porous matrix, surface
combustor-fluid heating apparatus 30 further comprises
fuel/oxidant mixture means for introducing a mixture of a
fuel and a primary oxidant into stationary porous bed 35 and
secondary oxidant means for introducing a secondary oxidant
into stationary porous bed 35. In accordance with one
embodiment of this invention, said fuel/oxidant mixture
means comprises fluid-cooled flow distributor 33. Said
secondary oxidant means for introducing a secondary oxidant
into stationary porous bed 35, in accordance with a
preferred embodiment of this invention, comprises at least
one secondary oxidant in~ection tube 44 disposed within
stationary porous bed 35 downstream of the primary
combustion zone defined by arrows 41.
As previously discussed, said porous bed heat
exchanger means comprises at least one fluid-cooled heat
exchanger tube 37 disposed in stationary porous bed 35. In
accordance with a particularly preferred embodiment, said at
least one fluid-cooled heat exchanger tube 37 is disposed in
the lower portion of stationary porous bed 35 at least about
1 inch to about 4 inches from flow distributor 33.
The process for staged combustion in porous
matrix, surface combustor-fluid heating apparatus 30 as
shown in Fig. 2 comprises introducing a fuel-rich,
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fuel/oxidant mixture into inlet end 31 of a combustion
chamber in which is disposed stationary porous bed 35. In
accordance with one embodiment of this invention, the fuel
is natural gas and the natural gas/oxidant mixture has a
stoichiometric ratio between 0.5 and about 1Ø Said
mixture is distributed in stationary porous bed 35 by means
of flow distributor 33, cooled by a cooling fluid flowing
through flow distributor cooling tubes 34 disposed within
flow distributor 33. From the distribution zone, defined by
arrows 39, of two-stage, porous matrix, surface combustor-
fluid heating apparatus 30, the fuel-rich, fuel/oxidant
mixture is preheated in a preheat zone, defined by arrows
40, disposed within stationary porous bed 35 immediately
downstream of distribution zone 39. From preheat zone 40,
the fuel-rich, fuel/oxidant mixture flows into the primary
combustion zone defined by arrows 41 immediately downstream
of preheat zone 40. Energy from the primary combustion zone
41 is radiated and conducted through stationary porous bed
35 including into preheat zone 40. Thus, the mixture of
fuel and oxidant is preheated in preheat zone 40 to its
ignition temperature by convective and radiative heat
transfer from high temperature ceramic particles 36 of
stationary porous bed 35. Due to the high turbulence levels
created by high temperature ceramic particles 36, rapid
partial oxidation of the fuel occurs within the primary
combustion zone 41. Products of incomplete combustion are
produced in primary combustion zone 41 having high levels of
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H2, CO, and N2. Extremely low levels of NOx and total
unburned hydrocarbons are produced in primary combustion
zone 41.
Energy from the combustion process in primary
combustion zone 41 is convected and conducted to lower
fluid-cooled heat exchanger tubes 37 disposed in the lower
portion of stationary porous bed 35. In addition, energy is
radiated from high temperature ceramic particles 36 to lower
fluid-cooled heat exchanger tubes 37. Lower fluid-cooled
heat exchanger tubes 37 rapidly remove heat by means of a
cooling fluid, for example water, flowing therethroUgh.
Secondary oxidant is then injected into a secondary oxidant
injection zone defined by arrows 42 in stationary porous bed
35 disposed adjacent to and downstream of primary combustion
zone 41. Secondary oxidant is injected into secondary
oxidant injection zone 42 through secondary oxidant
injection tubes 44 disposed within secondary oxidant
iniection zone 42 of stationary porous bed 35. The high
turbulence levels created by high temperature ceramic
particles 36 within stationary porous bed 35 enhance mixing
of the secondary oxidant with the products of incomplete
combustion from primary combustion zone 41, forming a
secondary combustion zone defined by arrows 43 in stationary
porous bed 35 disposed immediately downstream of secondary
oxidant injection zone 42. The combustion efficiencY of the
products of incomplete combustion and secondary oxidant is,
therefore, enhanced. In accordance with a preferreti
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embodiment of the process of this invention, the amount of
secondary oxidant introduced into secondary oxidant
injection zone 42 is sufficient to complete com~ustion of
the products of incomplete combustion from primary
combustion zone 41.
To maintain the flame temperature within
stationary porous bed 35 below the temperature required for
formation of thermal N0x, the heat generated by completion
of the combustion reaction in secondary combustion zone 43
is rapidly removed by upper fluid-cooled heat exchanger
tubes 45 disposed in secondary combustion zone 43 of
stationary porous bed 35. Because a significant amount of
energy, about 10~ to 30%, is removed from primary combustion
zone 41 through lower fluid-cooled heat exchanger tubes 37
disposed in primary combustion zone 41, the flame
temperature in upper fluid-cooled combustion zone 43, upon
completiOn of combustion and additional rapid heat removal
through upper fluid-cooled heat exchanger tubes 45 will be
reduced significantly below 2800F. Thus, the formation of
thermal N0x is reduced. In addition, formation of prompt
N0x is also reduced due to the presence of very low levels .
of unburned hydrocarbons in the products of combustion from
primary combustion zone 41. By reducing the formation of
NOX in primary combustion zone 41 and thermal and prompt NOX
in secondary combustion zone 43, total N0 levels from two-
stage, porous matrix, surface combustor-fluid heating
apparatus 30 can be less than 10 vppm.
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Experimental and theoretical data both indicate
that low NOx, less than 1.0 vppm at 3% 2~ will be produced
in the first stage of the two-stage porous matrix-surface
combustor fluid heating apparatus 30 in accordance with this
inventiOn, and less than 10 vppm of NOx is produced after
complete combustion in the second stage. Levels of CO and
total unburned hydrocarbons are accordingly less than ZO
vppm and 5 vppm, respectively. In addition, heat exchanger
tube fluxes exceedin~ 90,000 Btu/hr-ft2 have been
demonstrated.
While staged combustion with heat removal for NOx
reduction in industrial processes is known, the two-stage
porous matrix surface-combustor fluid heating apparatus and
process for combustion of a fuel therein in accordance with
this in~ention are unique in that high heat transfer surface
areas are produced by high temperature ceramic particles 36
compriS-n~ stationary porous bed 35 in both primary
combustion zone 41 and secondary combustion zone 43. This,
in turn, increases heat removal rates by means of enhanced
radiant heat transfer which, in primary combustion zone 41,
reduces the overall flame temperatures which reduces the
formation of thermal NOX therein, and in secondary
combustiOn zone 43, further reduces the overall flame
temperature~ which further reduces the formation of thermal
NOx. In addition, fluid tube heat fluxes associated with
lower fluid-cooled heat exchanger tubes 37 and upper
fluid-cOoled heat exchange~ tubes 45 in primary combustion
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zone 41 and secondary combustion zone 43, respectivelY, are
increased by means of enhanced convective and radiant heat
transfer, thereby enabling reduction in the size of tubes
required and resulting in improvement of overall thermal
efficiency.
In addition, the turbulent flow produced by high
temperature ceramic particles 36 in both the primary
combustion zone 41 and secondary com~ustiOn zone 43, in
addition to increasing heat removal rates and increasing
fluid tube heat fluxes by means of enhanced convective heat
transfer, also improves combustion intensity, thereby
concentrating heat release from the combustion process in
~oth the primary combustion zone 41 and secondary combustion
zone 43. The turbulent flow produced by high temperature
ceramic particles 36 in stationary porous bed 35 further
results in homogeneous combustion which reduces peak flame
temperatureS and the formation of pockets of high Oz
concentrations~ thereby reducing the formation of thermal
NOx in both primary combustion zone 41 and secondary
combustion zone 43. The turbulent flow produces efficient
conversion of substoichiometric oxidant and fuel mixtures in
primary combustion zone 41 to high hydrogen and CO
compositionsl that is products of incomplete combustion,
with very low hydrocarbon concentrations, thereby reducing
the formation of prompt NOx. The turbulent flow also
produces complete and homogenous mixing of the incomplete
combustion products from primary combustion zone 41 with
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secondary oxidant in secondary combustion zone 43 at near
stoichiometric levels, thereby also reducing reduced peak
flame temperatures and the generation of pockets of high 2
concentrations, both of which reduce thermal N0~ formation.
In accordance with one embodiment of the two-stage
porous matrix, surface-com~ustor fluid heating apparatus of
this invention, high temperature ceramic particles 36
comprising stationary porous bed 35 are coated with a
combustion catalyst. In accordance with another embodiment
of this invention, lower fluid-cooled heat exchanger tubes
37 and/or upper fluid-cooled heat exchanger tubes 45 are
coated with a combustion catalyst. Coating of high
temperature ceramic particles 36 and/or fluid-cooled heat
exchanger tubes 37, 45 allows operation of the two-stage
porous matrix, surface-combustor fluid heating apparatus at
stoichiometric ratios in primary combustion zone 41 less
than 0.5, further reducing the N0x formation in the first
stage, and reduces the formation of unburned total
hydrocarbons in primary combustion zone 41, thereby reducing
N0x formation in secondary combustion zone 43.
In accordance with yet another embodiment of this-
inventiOn, a diluent such as recirculated flue gases
nitrogen and/or steam, is injected into primary combustion
zone 41 and/or recirculated flue gases, excess air,
nitrogen, and/or steam are injected into secondary
combustion zone 43 to further reduce the peak flame
temperatures in these zones. The diluent can be injected by
IGT-1337/1293-B 20 10/3
mixing with the primary fuel/oxidant mixture and/or with the
secondary oxidant.
While in the foregoing specification this
invention has been described in relation to certain
preferred embodiments thereof, and many details have been
set forth for purpose of illustration, it will be apparent
to those skilled in the art that the invention is
susceptible to additional embodiments and that certain of
the details described herein can be varied considerably
without departing from the basic principles of the
invention.
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